Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologyhardmcq1 pt

Cyanide binds cytochrome c oxidase (complex IV) and stops electron transfer to oxygen. Which downstream effect follows most directly?

A.Proton pumping halts, the existing gradient dissipates, and ATP synthesis collapses
B.Glycolysis accelerates enough to fully replace the missing mitochondrial ATP supply
C.The Krebs cycle speeds up to rebuild the gradient using extra acetyl-CoA
D.Oxygen accumulates in the matrix and reacts directly with ADP to form ATP

Explanation

Core Concept

With complex IV blocked, electrons pile up along the chain and can no longer pass to oxygen, the final acceptor. The complexes stop pumping protons because electron flow through them ceases. Without continued pumping, the proton-motive force leaks away and ATP synthase loses its driving gradient. Tissues then depend on fermentation alone, which explains cyanide's rapid toxicity.

Correct Answer

AProton pumping halts, the existing gradient dissipates, and ATP synthesis collapses

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